I need to dot fill line in math mode even in `aligned`

environment. I'm using `\makebox[width]{\dotfill}`

but for presentation accommodations I want that dots reach the end of line.

I know that there exist `linegoal`

to compute the missing space but I can't find how to apply this in math mode. Is there anyway to get the missing space.

```
\documentclass[a4paper,12pt]{article}
\usepackage{multido}
\usepackage{amsmath}
\usepackage{lipsum}
\begin{document}
\lipsum[8]\parskip 1\baselineskip
\noindent
$f(x)=\frac{1}{\sqrt{x^7}}\Rightarrow
\begin{aligned}[t]
f'(x) & =\makebox[.6\linewidth]{\dotfill} \\[1.5\baselineskip]
& =\makebox[.6\linewidth]{\dotfill}
\end{aligned}$
\end{document}
```

## Best Answer

Since it seems you're interested in using the full width of the text block, you might as well use another mechanism to set the equation. Below I've used

`tabularx`

as it allows for a`p`

-type column that stretches as far needed to fill (say)`\textwidth`

: